Ancient zircon contains the secrets of our cosmos

Tom Chivers

It's a grain of sand, really. A grain of sand, found in the dust of the Jack Hills. But it's a grain of sand that has seen continents rise and fall. It was already ancient when the dinosaurs were destroyed. For that matter, it was ancient when the dinosaurs became dinosaurs. It was ancient when the first things came out of the seas, and when the first fish started swimming in them. This grain of sand is so old that it is nearly three times as old as all complex life, and a fair bit older than life itself.

A 4.4 billion-year-old zircon crystal from the Jack Hills region of Australia.

The grain of sand is a tiny crystal of zircon, a kind of semi-precious mineral comprising the elements zirconium, silicon and oxygen, usually used as a diamond substitute in cheap jewellery. It is 4.4 billion years old. When it was formed, the Earth was barely born, a planetary whippersnapper of 100 million years, still being pummelled by the debris floating around in the nascent solar system.

What's especially interesting about zircon, though, is not how old it is, but how we know how old it is, and what that, in turn, tells us. When zircon forms, crystallising out of cooling magma or in hot, high-pressure rocks, its rigid cubic crystal structure traps atoms of a particular kind of uranium, uranium-238. Just a few, a tiny few; maybe a dozen atoms for every million in the crystal. But that's enough. Because uranium-238 decays into lead at a predictable and fantastically slow rate - half of its atoms every 4.47 billion years - scientists can look at the ratio of uranium to lead in the crystal and work out how long ago the crystal was formed. The Jack Hills crystals have been known about for over a decade, but a new piece of research has confirmed their age.

The timeline of the history of Earth, which places the formation of the Jack Hills zircon and a "cool early Earth" at 4.4 billion years. Photo: University of Wisconsin

If you think of the very early Earth, you probably think of a hellish ball of lava. And, in fact, the period when this grain of sand was made is called the "Hadean aeon", the hell time. But the simple fact that this piece of zircon existed in this period tells us something interesting: there was rock. There was a crust to the Earth, it wasn't simply a great boiling liquid ball. And, as Adam Rutherford says in his book Creation, the particular kind of oxygen found in the Jack Hills zircons suggests that there was liquid water on this baby Earth; that already it was cool(ish), suitable for life.

Finding the history of the Earth in a grain of sand - delving back into the story of the beginning of our planet, or the universe, or life - requires this sort of ingenuity. It's not like determining that Captain Cook visited Botany Bay in 1770; there are no written accounts of the Big Bang, no diaries kept by humanity's last common ancestor with apes.

Advertisement

To learn these things, scientists have used clever proxies. One of the first attempts was by Lord Kelvin, the great early thermodynamicist, who tried to determine the age of the Earth by working out how long it would take for a planet-sized body to cool down from liquid heat to its current temperature. (He got it badly wrong, because he didn't know about radioactivity. But it was a nice idea.)

You will now receive updates fromTechnology Newsletter

Technology Newsletter

The history of science is full of these tricks. When it was noticed that all the galaxies in the universe were flying apart from each other, researchers realised that they could play the tape backwards, so to speak, and work out when they were all in one place, and a date was estimated for the Big Bang; in 1958, a man called Allan Sandage did this, and got a figure of 13 billion years. More than half a century later, using satellite measurements of the background heat of the universe and other hi-tech methods, Sandage's successors have managed to show that he was basically right: the universe is now known to be 13.8 billion years old, give or take a hundred million.

Millions of years, billions of years; my imagination can't grasp these numbers, except as a vague, unsettling feeling of standing on a precipice over a chasm of nothingness. I didn't exist for 99.999999997 per cent of history; in a few short decades, I won't exist for the rest of it. But there is something magnificent about it all, about the brilliance of the men and women who have worked out these incomprehensible figures, and about the idea that humanity, in its brief firefly-flicker of existence, has managed to reveal so much about the cosmos and its history. We have become the part of the universe which understands itself, however imperfectly, however dimly. The little zircon crystals, these sands of deep time, are one more tiny window into the history of the ancient Earth.